1. Field of the Invention
This invention relates to an integrated circuit adapted to perform the function of a DIAC diode.
2. Discussion of the Related Art
As is known, there are many applications, especially where oscillations are required to be triggered in a system of the oscillating type (e.g. an auto-oscillating half-bridge system for so-called "lighting" applications), for which diodes of the DIAC (Diode Alternate Current) type are useful. These diodes are multi-junction bipolar devices, e.g. of the NPNPN type, which are used in applications either as discrete components or integrated components.
Current integration technologies for electronic devices have, however, a disadvantage in that they do not allow of a simple and inexpensive integration of DIACs. As applied to oscillating systems, the DIAC has to produce a pulse for triggering the system oscillations, which then deactivates itself.
Referring to FIG. 1, which shows a graph of a voltage-current characteristic of a diode of the DIAC type, it can be seen that this diode has a very high impedance until the value of a voltage Vd thereacross attains the value of a predetermined diode-triggering voltage indicated at V1 in FIG. 1. Upon the voltage Vd reaching the triggering voltage V1, the diode becomes conductive and behaves almost as a short circuit, the current through it being only limited by its intrinsic resistance.
Again with reference to FIG. 1, it can be seen that as the value of the voltage Vd drops below the value of a so-called sustaining voltage, indicated at Vs on the graph, the diode is no longer conductive and reverts to a condition of high impedance.
Shown in FIG. 3 is an example of application of a diode of the DIAC type to an electronic device for driving a fluorescent lamp. This device, known to the skilled in the art as the "lamp ballast", is an auto-oscillating half-bridge device.
Referring to FIG. 3, the electronic device, indicated at OSC, comprises a transformer having a primary winding P connected in series to a fluorescent lamp TUBE, an inductive load L, and a first capacitor C1. The device OSC further comprises first SW1 and second SW2 switches connected in series between first and second terminal of a supply voltage generator Vr. The two switches SW1 and SW2 are connected in series together through a first intermediate node A. The first switch SW1 and the second switch SW2 are also connected in parallel respectively to first S1 and second S2 secondary winding of the transformer.
The device OSC further comprises a trigger circuit INN including first R1 and second C2 circuit elements which consist of a resistor R1 and a second capacitor C2, respectively, connected in series between the first and second terminals of the supply voltage generator Vr. The resistor R1 and second capacitor C2 are connected in series together through a second intermediate node B. The trigger circuit INN also comprises a first diode D1 of the DIAC type having a first terminal connected to the second intermediate node B and a second terminal connected to the switch SW2 to perform a triggering function for the oscillations of the device OSC. A second diode D2, connected with first and second terminals to the first A and second B intermediate nodes, respectively, functions to disable the first diode D1.
In practice, the device OSC is triggered by the first diode D1 receiving a current from the series connection of the second capacitor C2 and the resistor R1, and produces a first pulse which is received by the switch SW2.